This invention relates to expansion joints or so-called compensators for establishing flexible joints in pipes, conduits, ducting and the like, hereinafter referred to simply as pipes. Such compensators may comprise a sealing element of plastics or rubber, having limited thermal resistance, and one or more flexible layers of textile or like material.
Known compensators of this nature are usually assembled from several layers of asbestos fabric and/or other industrial fabric, in which a sheet or foil of polytetrafluoroethylene (PTFE) is embedded in order to obtain the required gas-tightness. In thermal load-bearing capacity and also in cost incurred, such compensators of pliable material lie between the also-known steel compensators and rubber compensators. Steel compensators, which are welded up from steel sheets and are self-supporting, are primarily appropriate for high pressures and also for liquid fluids. Their high cost, however, precludes their application from many uses and, furthermore steel compensators have a relatively very low degree of flexibility. On the other hand, rubber compensators are satisfactory if the pipe cross-sections are circular and if they can be produced in bulk. The thermal resistance is low, however, and rubber compensators are also unsuitable for many purposes because of their lack of chemical resistance.
In a compensator of pliable material, the sealing element functionally represents the essential component. It is the principal purpose of the flexible layers of fabric material to protect the sealing element which usually consists of a sheet of PTFE, aluminium, or lead, against temperature, pressure, and mechanical stress, without the desirable flexibility of the compensator being lost. At the present time, asbestos and/or synthetic fabrics may be considered as materials which combine flexibility, thermal stability and chemical imperviousness, to the degree required for most purposes. Fabrics of this nature tend to decay, however, for example in cases where there is substantial condensation. Also, in installing the pipe systems, the fabric may be damaged since it is the external layer of the compensator. Moreover, the fabrics may not protect the sealing elements adequately in cases of intermittent thermal loads from inside, the fluid side, or arising from external heat radiation. In such cases the compensators may leak.
It has already been proposed in German Pat. No. 1,273,929 to eliminate the above shortcomings of conventional compensators of pliable material by making provision for high thermal stability in the sealing element itself. In this proposal, a self-supporting steel sheet insert having a thickness of up to 0.2 mm was incorporated as a sealing element which was inserted between the fabric layers directly exposed to hot gas. Practice showed that the sheet or foil thickness should not lie appreciably below 0.2 mm, in order to obtain seal-supporting qualities, but self-supporting steel sheets are so inflexible that the compensator required a bellows formation. The production of bellows is costly, however, particularly in view of the large cross-sections which are involved in steel compensators. An even greater disadvantage is that the steel sheet insert must be welded in a gastight manner, since it forms the sealing element. The assembly and installation of such compensators are thereby rendered considerably more difficult and, in numerous cases, practically impossible.